8.39 Compressed air at p = 20 atm enters a 20-mm- diameter tube at Tmi = 20C and a mass flow rate of 610 kg/s. The air is warmed by a constant surface heat flux so that its exit mean temperature is m = I'm.o 50C. Determine the required surface heat flux and the temperature of the tube wall at the tube exit for L=0.15, 1.5, and 15 m.